The present invention relates to a magnetic recording and reproducing apparatus used for an auxiliary storage of a computer and magnetic recording media used for it and more particularly to perpendicular magnetic recording media suitable for realizing a high recording density of 4 giga bits or more for a square inch and a magnetic recording and reproducing apparatus using them.
Due to progress of the information-oriented society, the information amount which is handled daily keeps on increasing. Along with it, there is an increasing need for a high recording density and a large storage capacity of magnetic recording apparatuses.
When the density of a magnetic recording apparatus is increased, the medium area for each recording bit is decreased, so that the reproduced output is lowered and the reproduction becomes difficult. To solve this problem, a method that recording and reproducing are executed by different heads and a head using a magneto-resistive effect having a high sensitivity is used as a reproduction head is used practically. To make the density higher, a head using a giant magneto-resistive effect having a higher sensitivity is examined. By use of such a highly sensitive reproduction head, the reproduced output can be increased, though the noise is also amplified at the same time and when a medium having a high noise is used, recorded information cannot be read. Therefore, for a magnetic recording medium for recording and reproducing data with high density, it is necessary to keep the medium noise lower.
In a longitudinal magnetic recording system used for a current magnetic disk, to reduce the medium noise, it is necessary to make crystal grains finer. Furthermore to overcome the demagnetizing field from the magnetic charge at the bit boundary and hold the magnetization in the recording direction, it is necessary to increase the coercivity, reduce the product of the thickness of the magnetic layer and the remanent magnetic flux density at the same time, and make the demagnetizing field smaller. To solve such a problem and achieve an areal recording density of 4 giga bits or more for a square inch, it is necessary to realize a coercivity of 3000 to 4000 oersted with a thickness of 10 to 20 nm of the magnetic layer and it is expected that the realization is technically difficult considerably. When crystal grains are made finer and the thickness of the magnetic layer is reduced, the thermal stability of the recorded magnetization state also becomes the matter.
On the other hand, the perpendicular magnetic recording system is characterized in that as the recording density increases, the demagnetizing field reduces and furthermore the thickness of the magnetic layer can be increased, so that the perpendicular magnetic recording system is considered to be a system suited to high density recording because the recorded magnetization state is thermally stable. To execute high speed recording and reproduction, even in the perpendicular magnetic recording system, it is a necessary condition to reduce the medium noise. Noise in perpendicular magnetic recording media is considered to depend on the size of the reversed magnetic domain in the recording bits and the size of disturbance at the recording bit boundary. To make those sizes smaller and reduce the noise, it is necessary to reduce the magnetization reversal unit by making the diameter of crystal grains of the magnetic film smaller.
The size of magnetization reversal unit is related also to the magnetic viscosity. Namely, it is considered that as the fluctuation field of magnetic viscosity increases, the magnetization reversal unit reduces and the medium noise also reduces. The meaning of the fluctuation field of magnetic viscosity is described in Journal of Physics F: Metal Physics, Vol. 14, p. L155 to L159 (1984). It is difficult to compare noise values of various media quantitatively and even in the same medium, the noise value to be measured varies with the head kind and the mutual relationship between the head and the medium. On the other hand, measurement of the fluctuation field of magnetic viscosity is simple and reproducible. It is described in Journal of Magnetism and Magnetic Materials, Vol. 127, p. 233 to 240 (1993) in detail.
Perpendicular magnetic recording media have been researched and developed mostly about continuous thin film type magnetic tapes. In this case, the thickness of the magnetic layer is as thick as 100 nm or more and a head with a wide track width is used for recording and reproduction, so that the reproduced output is large and it is not necessary to suppress the medium noise level so much. On the other hand, when a perpendicular magnetic recording medium is used as a magnetic disk, it is necessary to increase the density also in the track direction, so that the area of the recording bits reduces and a small output is reproduced by a highly sensitive head. As a result, the restriction to the medium noise becomes strict necessarily. Examination results relating to noise of perpendicular magnetic disk media are described in, for example, Journal of Magnetism and Magnetic Materials, Vol. 134, p. 304 to 309 (1994), which indicates that with respect to a CoCrTa perpendicular dual-layered film medium, medium S/N at 90 kFCI is 23.8 dB. Therefore, recording and reproduction with a high areal recording density of 4 giga bits or more for a square inch are considered to be difficult.
To increase medium S/N of a perpendicular magnetic recording medium, various films in which only the thickness of the magnetic layer is changed are manufactured by way of trial and the magnetic characteristic and the noise characteristic are evaluated by a vibrating sample magnetometer and a magnetic resistance effect type head respectively. Evaluation results are as shown below. When the film thickness is reduced so as to make the diameter of crystal grains smaller and reduce noise, the noise reduces almost in proportion to the film thickness, though the coercivity also reduces suddenly. As a result, the reproduced output reduces and a medium having a satisfactory S/N characteristic cannot be obtained. When the film thickness is increased so as to increase the output, the output tends toward saturation and does not increase so much, though the noise increases in proportion to the film thickness and S/N lowers. When the thickness of the magnetic layer is within a range from 50 nm to 150 nm, medium S/N shows a relatively high value but does not reach a level for reproducing a record with a high density of 4 giga bits or more for a square inch and reading information.